Copper Film Enables Accurate Optical Humidity Sensing

Today’s food packaging gives you a lot of information about your food, including ingredients and number of calories, but it cannot tell you whether the fruit salad you want to buy is drying out or if your breakfast cereal is crunchy or soggy.

The freshness of many consumer goods often depends on the humidity inside the packaging. However, incorporating humidity sensors into consumer packaging is currently not feasible because conventional sensors require electronics, a power source and a display.

A humidity sensor being developed at Swiss Federal Institute of Technology (ETH) in Zurich, however, does not require any of these. The device is simply a copper-based strip that changes color when a certain humidity level is reached.

The sensor material could be applied onto the package itself, according to Norman A. Lüchinger from the Institute for Chemical and Bioengineering at ETH. It is based on carbon-coated copper nanoparticles suspended in a polymer matrix to create a stable water dispersion. During preparation of copper dispersions, the researchers found, almost by accident, that they showed strong and vivid coloration patterns when in contact with humid air.

Not only is this new sensor simpler than conventional devices, but it also is more accurate. Commercial low-cost humidity sensors are not known for their accuracy and often exhibit absolute errors of at least ±1 percent in relative humidity, Lüchinger said. The absorbance maxima of the copper films developed at ETH shift by 50 nm for every 1 percent change in relative humidity, showing a strong color change for small variations in humidity.

The researchers believe that this phenomenon can be attributed to a combination of surface plasmon resonance effects and thin-film interference, depending on the humidity in the air. Other nanotechnology-based humidity sensors under investigation, such as colloidal crystals, photonic bandgap composites or opal hydrogels, use a regular arrangement of monodispersed particles with diameters suitable for Bragg diffraction.

The ETH sensor, on the other hand, uses smaller particles in an irregular arrangement that cannot show Bragg-type diffraction within the film.

At low humidity (less than 70 percent), the polymer matrix of the dry film takes up water, which increases its volume and shifts the surface plasmon resonance peak. When the humidity is high, additional vapor condenses on top of the porous film, causing interference and a particularly strong color effect. The work is detailed in a Langmuir paper published March 13.

A simple sensor based on carbon-coated copper nanoparticles (a) shows a dramatic change of color when exposed to ethanol vapor (b). When exposed to water vapor (c), it changes color from green and gradually shifts back to pink (d), blue (e) and orange (f) under ambient humidity. Images reprinted with permission from Langmuir.
According to Lüchinger, changing the sensing material and including additives could be a promising approach to novel dispersions that react differently to various levels of humidity. “This means that, by making an array of copper fields, an accurate humidity reading can be made optically. If this array of sensor fields can be calibrated, the sensor can also be used to give [a] quantitative humidity reading.”

At low humidity, the change of color is the result of surface plasmon resonance effects as the polymer matrix swells when in contact with water. At high humidity, the change of color is the result of thin-film interference.
Tests have shown that the sensor material can be used several times because the coloration effect is completely reversible. The same material also can be used to detect other vapors, such as ethanol and acetone. This opens up further uses for the sensor, including industrial applications where moisture detection is essential for product quality, along with visualization of gas fluid dynamics and detection of organic solvent vapors.

“We are keen to commercialize the sensor, but more detailed studies have to be carried out on the temperature dependence of the coloration and the choice of polymers,” Lüchinger said.

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